Checking low-water cutoffs


Service techs must know the correct procedure to complete this common task

By George Lanthier

I love surfing the Internet and the information that’s available there is incredible. There are several great sites for heating guys and we are finally getting around, by request, to bringing back our own Chat Room, DragonTalk. My hopes are that it will not only house a ton of information on oilheat, including a library of my old articles, but will also be fun and another contribution to industry education. It will be all oilheat, with no pop-ups, ads or other distractions.

A while ago a thread started on the Internet about checking low-water cutoffs that got me thinking about whether or not most techs know the correct procedure for doing this common task. In my steam and control seminars, I know most techs are very upset to find out they don’t and so, here we go.

ASME approval logos, which are a four-leaf clover with a letter in the middle, are approvals given to boilers, valves and other devices legally called ‘appurtances” that are found on steam or water boilers. ASME is the American Society of Mechanical Engineers and having been a member since 1976 I tend to read an awful lot of their publications when they come to my door. That letter in the middle of the clover leaf means something, too. If it’s an ‘H,” it means you’re looking at a cast-iron boiler or section and if it’s a steel boiler or tank. the plate is welded on. If it’s an ‘HV,” that stands for hydronic valve and an ‘N” would mean you’re in the wrong place and looking at a reactor.

ASME has inspectors who not only approve this stuff after it’s made, but actually work in many foundries and weld shops to check it while it’s being made. The basic two manuals that boiler people live by are ASME Section IV, which is a pretty big book, and CSD-1, which is a standard for controls that operate the boilers. It’s these two books that get pulled out in court and if you’ve messed up it’s the two that will hang you. Forget NFPA standards, good industry practice and a bunch of code and textbooks, it’s these two bad boys that rule the roost around boilers.

Anyway, with the background out of the way, let’s look at checking a steam low-water cutoff and we’ll start with the older float-type first. How do you check one of these? Do you get the burner running and drain the cutoff until the burner shuts off? Well, if you do, you’re doing it wrong. All you’re doing is flushing the cutoff and the burner happens to go out. How do you know that all of the cutoff piping is clear? What if one of the sections is plugged? On commercial equipment, what if a mud-drum is plugged? The only correct way is to get the burner running and draining the boiler until the burner shuts off, according to ASME, and that’s a fact.

The same test is true for probe-types, but in addition, you must also verify that the probe is clean and check the cutoff for operation. That doesn’t mean checking it by pushing a ‘test button.” C’mon you’re supposed to be the pro on the job and this is a ‘routine or annual check,” not a quick test. Clean the probe according to the OEM’s recommendations, don’t use Teflon when putting it back in. Teflon is an insulator and make sure that the sensor doesn’t come within a quarter of an inch from any metal in the boiler or piping.

On a water boiler, this whole procedure could become a P-I-T-A with a three- or six-story building above you. So, I’ll give you a couple more ideas. The first is a test procedure from my freinds at Hydrolevel that is another unscientific way of checking a probe.

Take the wire off the probe and turn on the burner; it should not start. Now, turn off the power again. Take the probe wire off the probe and place it on the case of the cutoff, Figure 1. The burner should immediately start.

Finally, here is a test procedure using a meter and a little rig that was originally developed by Honeywell for their old Guard-Ring Series. The rig you see in Figure 2 was made up by my buddy, Mr. Wizard, Jim Todd of the ICPA in Connecticut. It consists of the materials shown in Figure 3 and essentially proves the probe is working by passing a small, 9-volt DC charge through the water and back to the probe.

You can make this rig up for about five bucks with parts from a store like Radio Shack and all it takes are the following: a 9-volt battery, a 9-volt battery power lead, a 3900 ohm resistor, two meter plugs, a jumper wire and the box is made from a cassette tape storage case. Make it up and use it as shown in Figure 3 and you have a great way to CYA and get the job done. So, if you ever get dragged into a deposition or have to give testimony on how to check a low-water cutoff, now you know the ‘correct way.”

By the way, hopefully I’ll see you on DragonTalk. I’ll have to go there to stay away from e-Bay. I’m a tool and collectible toy freak, so e-Bay is not good for my wallet. When it comes to tools, it’s just about anything, but with collectibles it’s gotta be a tank truck or fire engine. I bet you’re surprised?

See ya.

George Lanthier is the owner of Firedragon Enterprises, a teaching, publishing and consulting firm. He is an industry trainer and the author of nine books on oilheating and HVAC subjects. He can be reached at 132 Lowell Street, Arlington, MA, 02474-2756. His phone is (781) 646-2584, fax at (781) 641-7099 and his Web site and chat room can be found at

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